摘要:

The singles and doubles coupled-cluster method that includes a perturbational estimate of the effects of connected triple excitations, CCSD(T), together with a triple zeta double polarized (TZ2P) one-particle basis set is used to determine the geometries, harmonic frequencies, infrared intensities, and dipole moments of HOF, F2O, HOOF, FOOF, C1OOF. Agreement with experiment is very good, with the exception that the currently accepted experimental assignment of the symmetric and antisymmetric O–F stretches in FOOF is shown to be reversed (and to be consistent with an earlier experimental study). Very accurate heats of formation of HOOF, FOOF and C1OOF are also computed using the CCSD(T) method in conjunction with large atomic natural orbital basis sets. The F–O bond distances, quadratic force constants, bond energies, and fluorine and oxygen atomic charges from the above five molecules and six previously studied molecules (FONO2,trans-FONOcis-FONO, FOC1, FOBr and FON) are compared and used to deduce a simple model of F–O bonding. The unusual relationship between the F–O bond distance and quadratic force constant shows that F–O bonding is a function of at least three effects, which are degree of covalent character, degree of ionic character, and extent of lone electron-pair repulsions. All of the data are qualitatively consistent with this simple model The bonding incis-FONO is even more complicated, involving also dispersion interactions between fluorine and the terminal oxygen. It is suggested that the general importance of lone pair repulsions in F–O bonding and the additional importance of intra-molecular dispersion interactions explains why many density functionals have difficulty in describing the geometry ofcis-FONO.

ISSN:0026-8976    

DOI:10.1080/002689796173237

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

We report the measurement of far-infrared vibration–rotation tunnelling parallel bands of two partially deuterated water trimer isotopomers: (D2O)2DOH and (H2O)2DOH at 97•2607 cm-1and ∼ 86 cm-1, respectively. The hydrogen bond rearrangement dynamics of the two mixed trimers can be described by the simplified molecular symmetry G8, which accounts for both the flipping and bifurcation tunnelling motions previously established for (H2O)3and (D2O)3. The observed donor tunnelling quartet, rather than triplet, splitting indicates that the two homogeneous monomers (D2O or H2O) in each mixed trimer experience slightly different environments. Vibrationally averaged structures of (H2O)3, (D2O)3, and (D2O)2DOH were examined in a Monte Carlo simulation of the out-of-plane flipping motions of the free atoms. The simulation addresses both the symmetric top behaviour and the negative zero-point inertial defect for (H2O)3and (D2O)3, which were insufficiently counted in all previous structure models. The average ground state O—O separations, which are correlated to other angular coordinates, were determined to be 2•84±0•01 Å for all three species. The simulated difference in hydrogen bond nonlinearity also supports the inequivalency of the two homogeneous mono mers The structural simulation shows that the unique H in (D2O)2DOH is free, while a torsional analysis suggests the unique D in (H2O)2DOH is bound within the cyclic ring. Both bands can be assigned to the pseudorotational transitions which correlate to those found in the pure trimers.

ISSN:0026-8976    

DOI:10.1080/002689796173246

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

The motion of electron spins in anisotropic systems described by spin Hamiltonians containing zero-field tensors is described in terms of semi-classical trajectories constructed either in spin space from surfaces of constant energy or in energy space from surfaces of constant magnitude of the spin angular momentum. The trajectories are presented as time-independent projections for a range of values of the non-axial orthorhombic spin-Hamiltonian parameterE. Analogies are made to the closely related semi-classical description of the rotational motion of spherical symmetric and asymmetric top molecules. The spin-Hamiltonian parameters of a number of organic and inorganic systems are used to illustrate the results.

ISSN:0026-8976    

DOI:10.1080/002689796173255

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

Calculations of ground- and excited-state polarizabilities and hyper-polarizabilities ofcis-, trans-and diphenyl-polyenes have been carried out by means of cubic response theory implemented at the random-phase approxi mation level. The length, frequency and conformational dependences of these quantities are predicted and discussed.

ISSN:0026-8976    

DOI:10.1080/002689796173264

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

It is demonstrated that accurate values of the parameters, such as relaxation times, which determine the appearance of transient ESR spectra and, to some extent their time-dependence, can be obtained from analysis of the oscillation patterns observed at high microwave field strengths. A new methodology is introduced for this, which also allows more accurate determination of hyperfine coupling constants. This opens the route to precise simulation of polarized spectra and to the identification of the radical pairs from which the polarization results.

ISSN:0026-8976    

DOI:10.1080/002689796173273

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

A Hartree–Fock and post-Hartree–Fock study on thecis–transequilibrium of fluoroformyl hypofluorite, FC(O)OF, is presented. Pople's split-valence double- and triple-zeta quality basis sets, augmented with sets of polarization and diffuse functions (6-31G(d), 6-311G(d), 6-311G(2df) and 6-311+G(2df)), correlation consistent polarized valence triple-zeta Dunning's (aug-cc-pVTZ), and medium-size polarized Sadlej's (MSPBS) basis sets were used at the HF and at the MP2, MP4(SDQ), MP4(SDTQ), CCSD, CCSD(T), and DFT correlated levels. For all applied methods,trans-FC(O)OF is predicted to be more stable than thecisisomeric form, in agreement with experimental data. ΔE(cis⇒trans) values range from – 1•70(CCSD/6-311G(d)) to –0•16 kcal mo1-1(DFT/6-311G(d)) and thecis–transbarrier height ranges from 9•60 (MP2/6-311G (2df)) to 8•56 kcal mo1-1(MP4(SDQ)/6-311+G(2df)). Reliable geometries and IR harmonic vibrational frequencies were obtained using the DFT model with the Becke–Lee–Yang–Parr exchange-correlation potential.

ISSN:0026-8976    

DOI:10.1080/002689796173282

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

A theoretical description of femtosecond transition-state dynamics is developed and compared with experimental results, with numerous examples. We focus on the population flow, employing a kinetic model, with the objective of studying the general trends and signal control parameters; we do not include coherence for the sake of acquiring a simplified picture with operational parameters. Different methods of detection are addressed, such as laser-induced fluorescence, depletion, absorption, and mass spectrometry. The response of the molecular system is derived for various types of dynamics with one or more rate determining processes. The significance of the relative time scales associated with the rates is explored and the effect on transients of the ratio of probing cross-sections is examined. Furthermore, the influence on the observed signal of the finite temporal width and shape of the laser pulses is quantified. Effects are discussed both in the unsaturated and saturated regimes of transition probabilities, demonstrating characteristic features of both regimes.

ISSN:0026-8976    

DOI:10.1080/002689796173291

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

Expressions for the vibrational optical activity of an independent anharmonic oscillator using the second-order contact transformation theory are presented. The relative importance of anharmonicity for the magnetic dipole and electric dipole transition moments is made transparent using the potential constants for Morse oscillator. At this level of theory, the anharmonic contributions appear to be relatively more significant for the magnetic dipole transition moments.

ISSN:0026-8976    

DOI:10.1080/002689796173309

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

The mechanism of the observed amine sensitization of nitromethane has been investigated by means of a density functional procedure (Becke-3 exchange, Lee, Yang and Parr correlation functionals and 6-31+G*basis set). Of the various possibilities examined, the most reasonable is that involving theaci(nitronate) ion in an activated form which can interact with the amine eventually to yield another amine and the nitrite ion. The key features of this process are firstly an activation barrier less than the C–NO2dissociation of nitromethane, secondly a net release of energy and thirdly products that can sustain the reaction.

ISSN:0026-8976    

DOI:10.1080/002689796173318

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

A detailed analysis of the influence of a solvent modelled as a continuum defined by its static and high frequency dielectric constants, ε and ε∞, respectively, on the force constants of a molecule in solution is proposed. It is applied to the computation of the solvent-induced frequency shifts in the vibrational spectrum of formaldehyde. The sign and the order of magnitude of the effect are correctly predicted at the scaled Hartree–Fock, MP2 or DFT levels. The computations performed on conveniently distorted systems allow us to separate the influence of the reaction field on the molecular vibrations from the influence of anharmonicity, by means of the geometry distortion induced by the solvent. The anharmonicity contribution appears to be the major contri bution to the overall effect.

ISSN:0026-8976    

DOI:10.1080/002689796173327

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor